In an alternating current (AC) circuit, a capacitor functions as a reactive component that stores and releases energy.

Capacitors are passive components that store electrical energy in an electric field established between two conductive plates. In an AC circuit, the voltage across the capacitor varies periodically, leading the capacitor to charge and discharge in response. When the voltage is positive, the capacitor accumulates charge, and when the voltage turns negative, it releases that charge. This continuous process of charging and discharging causes the capacitor to exhibit reactive behavior, opposing changes in voltage.

The opposition to voltage changes in a capacitor is quantified as capacitive reactance, measured in ohms ($\Omega$). Notably, the capacitive reactance of a capacitor decreases as the frequency of the AC signal increases. This characteristic allows capacitors to permit high-frequency AC signals to pass while effectively blocking low-frequency signals. As a result, capacitors are frequently employed in AC circuits for noise filtering and to block direct current (DC) signals.

Additionally, capacitors exhibit a phase relationship with the voltage in an AC circuit. Specifically, the voltage across a capacitor lags behind the current by $90^\circ$. This means that the capacitor stores energy when the current reaches its peak and releases energy when the current is at its minimum. Understanding the phase relationship between voltage and current is crucial for comprehending the behavior of capacitors and other reactive components in AC circuits.